This invention relates to a simplified process for the preparation of polyurethane urea elastomers in which solid, high-melting aromatic diamines are reacted with polyisocyanates or isocyanate prepolymers in a heterogeneous reaction.
The preparation of polyurethane urea elastomers from polyisocyanates, relatively high molecular weight polyhydroxyl compounds, and aromatic diamines is known. To guarantee reasonable processing times for reactive systems of such starting components, reactive aromatic isocyanates generally used on an industrial scale are preferably reacted with sluggishly reacting diamines. In practice, diamines that have been successfully used in this way are primarily aromatic diamines of which the basicity and, thus, the reactivity to isocyanates have been reduced by introduction of halogen or carboxy substituents. One example of such diamines is 3,3'-dichloro-4,4'-diaminodiphenylmethane ("MOCA") which has previously been the most widely used such diamine.
U.S. Pat. No. 3,891,606 discloses the crosslinking of isocyanate prepolymers of polyhydroxyl compounds and excess polyisocyanates with aromatic diamines in which the reactivity to isocyanate groups has been reduced by complexing with certain alkali metal salts. A disadvantage of this process is that it is confined to two special aromatic diamines. In addition, the complex between the aromatic diamine and the alkali metal salt must be prepared in a separate process step.
Another way to control the reaction rate between polyisocyanates and aromatic diamines is to carry out the reaction in an organic solvent. Processes of this type are disclosed, for example, in U.S. Pat. No. 3,926,922 and in Japanese 70/9195. A disadvantage of using organic solvents is obvious. The risk of fire and explosions is increased and the solvent must be recovered economically and ecologically in a further process step.
Before the present invention, little was known about the preparation of polyurethane ureas by reaction of polyisocyanates with aromatic diamines in heterogeneous phase. According to the prior art, aromatic diamines of relatively high melting point, which generally are of particular interest on an industrial scale, either are used in dissolved form, which involves the disadvantages just mentioned, or are reacted with polyisocyanates in the melt. The processing of aromatic diamines in the melt is described, for example, in U.S. Pat. No. 3,926,922 (mentioned above) or in German Auslegeschrift 1,122,699. German Auslegeschrift 1,122,699 relates to a process for the preparation of polyurethane elastomers by crosslinking liquid isocyanate prepolymers by reaction in molds with mixtures of primary diamines and compounds containing several hydroxyl groups. In this latter process, a dispersion of a powder-form crystalline diamine in a liquid polyester or polyether containing several hydroxyl groups or in castor oil is introduced into the prepolymer at a temperature below the melting point of the diamine. The mixture is cured as a melt by known methods at temperatures above the melting point of the diamine used in the mixture. In this process, therefore, the actual "amine crosslinking" reaction takes place in a liquid, homogeneous phase. A particular disadvantage of the process disclosed in German Auslegeschrift 1,122,699 is the need for the high temperatures which must be applied in the processing of high-melting diamines such as 1,5-naphthylenediamine (m.p. 189.degree. C.) or 4,4'-diaminodiphenyl ether (m.p. 186.degree. C.).
U.S. Pat. No. 3,105,062 discloses a process for the preparation of polyurethane ureas in which relatively high molecular weight preadducts containing isocyanate groups are reacted with preferably aromatic diamines in heterogeneous phase. The resultant reaction mixtures cure at a temperature at which the "two-phase system" changes into a "one-phase system". This temperature is generally in the range from 100.degree. to 170.degree. C. The aromatic diamines disclosed in U.S. Pat. No. 3,105,062, however, are soluble, albeit to only a limited extent, in the reaction medium (the NCO preadduct). Consequently, uncontrollable preliminary reactions take place during the mixing of the two components, even at room temperature, and the reaction mixtures thicken in a very short time and form partly paste-like formulations. These paste-like formulations are difficult to process by the standard casting method and, accordingly, must be brought into the required form by applying pressure before they are actually cured by heating. According to U.S. Pat. No. 3,105,062, the stability of the thickened reaction mixtures in storage (pot life) is sufficient for further processing (that is, molding under pressure and coating), amounting to several hours. It is apparent from the Examples that the preferred reaction mixtures are those having a maximum pot life of about one hour. Accordingly, these mixtures cannot be regarded as long-term systems. In addition, U.S. Pat. No. 3,105,062 specifically points out that the use of the disclosed diamines--present only in solid form--in the one-shot process leads to unsatisfactory polyurethane moldings. The unwanted preliminary reaction of the diamine with the diisocyanate takes place to an increased extent, the poorly soluble polyurea precipitating in the reaction mixture and no longer reacting.
German Offenlegungsschrift 2,635,400 discloses another process for the preparation of polyurethane urea elastomers in which aromatic diamines are reacted as chain-extending agents in a single-stage or multi-stage process. This process is characterized by the use of aromatic diamines having a melting point above 130.degree. C. that are present in the reaction mixtures in solid form. The heat curing of such mixtures takes place at a temperature in the range from 80.degree. to 120.degree. C., that is, below the melting point of the aromatic diamine. By virtue of the choice of the corresponding diamines as chain-extending agents, the NCO-containing preadduct (also referred to as an NCO prepolymer) is not involved in a premature preliminary reaction that results in thickening of the mixtures. Accordingly, systems of this type can be readily processed even by casting. Since the pot life of these reactive systems is considerably increased, many aromatic diamines, which were difficult to process by the previously known method, may be used in this process. It can be seen from the examples of German Offenlegungsschrift 2,635,400 that the pot life of the liquid reaction mixtures ranges from a few minutes to several hours, depending on the reactivity or solubility of the aromatic diamine. For standard processing conditions, for example, in the hand casting process, these reaction mixtures, particularly those having relatively long pot lives, can generally be processed without significant difficulties. In contrast, problems arise if, as a result of machine failures or other required stoppages, there is a relatively long interruption between the preparation of the reaction mixtures and the curing phase. Accordingly, the need for long processing times at low temperature and for short curing times at elevated temperature is increasingly more urgent in practice.
The final polyurethane plastics are generally intended to exhibit favorable mechanical properties and, in many cases, a level of thermal stability adapted to a particular application. According to the prior art, the thermal stability of polyurethane elastomers depends largely on the type of chain-extending agent used. For example, if glycolic chain-extending agents are used for the preparation of elastomers, the resultant polyurethane moldings have lower thermal stability than when using compounds containing amino groups. There are, of course, also distinct differences in thermal stability within the particular type of chain-extending agents (compounds containing OH or NH.sub.2 groups).
Accordingly, the object of the present invention was to find a process for the preparation of polyurethane ureas in which the starting components of the particular reaction systems (high molecular weight polyols or NCO preadducts and low molecular weight chain-extending agents containing NH.sub.2 groups and, optionally, other auxiliaries and additives) remain unreacted for several weeks at room temperature or, optimally, for at least 14 days at a temperature of about 50.degree. C. Such reaction mixtures may thus be regarded as "one-component systems" that cure only under the effect of relatively high temperatures. In addition, it is desirable that the mixtures that are capable of being cast at the processing temperature should be curable in economically useful reaction times.
The present invention is also based on the concept of finding suitable chain-extending agents containing amino groups which have only minimal solubility in the starting component (for example, in the NCO preadduct) at low temperatures but which have high solubility at relatively high temperatures, so that the polyurethane urea assumes a high molecular weight structure during the curing phase.
Another object of the present invention was to find a process for the preparation of polyurethane urea elastomers in which high-quality elastomers of high thermal stability are obtained.
It has now surprisingly been found that solid, high-melting diamines corresponding to the formula ##STR2## in which the NH.sub.2 groups are in the o-, m- or p-position to the ether oxygen and R.sup.1 and R.sup.2 represent hydrogen or alkyl groups (preferably methyl groups), give reaction mixtures that are stable in storage at room temperature when processed by the one-shot process or prepolymer process. Reaction systems such as these have a stability in storage of days to weeks at about 50.degree. C.
For compounds in which R.sup.1 and R.sup.2 are hydrogen and each NH.sub.2 group is para to the ether oxygen, one-component systems having indefinite stability in storage at room temperature or at elevated temperatures of up to about 50.degree. C. are obtained. A precondition in this regard is that the combination should be protected against the effect of atmospheric moisture in order to avoid unwanted reaction of the NCO groups with water.